In a flash memory device or a dynamic random access memory (DRAM) device, gate lines generally have elongated line shapes. Also, the gate lines are formed of silicon material having conductivity lower than that of metal material. Thus, voltages at portions, for example middle portions, of the gate lines remote from portions to which voltage is first supplied, become lower than the required voltage since the gate lines have line resistance. Accordingly, the memory device needs means to compensate for a drop in voltage in the middle portions of the gate lines. However, as the elements incorporated into a semiconductor device are integrated to a higher degree, a wiring width as well as a distance between cells becomes less and less, thereby increasing line resistance. In order to maintain voltage at all portions of the gate lines at a given level and to prevent an increased delay of the gate signal due to increased line resistance, means for restoring voltage are required. However, in most means for restoring voltage, a peripheral structure in the memory device is complicated, causing a loss of integration density. To reduce these problems, there have been proposed methods of increasing conductivity, such as forming silicon wiring of the gate lines by using a multi-layered silicon layer including a metal layer, or forming a metal silicide layer on an upper surface of each gate line as in a general flash memory device shown in FIG. 1 to FIG. 3
FIG. 1 is a top plan view showing a portion of a cell area of a general NAND type flash memory.
Referring now to FIG. 1, an isolation layer is formed on a substrate to form an active region 22 in a cell area. The active region 22 comprises a plurality of line shaped sub-regions which are defined respectively by a plurality of elongated openings or gaps of the isolation layer 23 shown in FIG. 2. In a center portion of the cell area, a common source line 45 is disposed to cross the active region 22. In each of upper and lower portions of the cell area divided by the common source line 45, a plurality of gate lines comprising a ground select gate line 33g, a plurality of, for example 8, 16, or 32 word lines WP, and a string select gate line 33s are formed in order from one of both sides of the common source line 45. Namely, two equal parts of gate lines formed in the upper and lower portion of the cell area are disposed symmetrically with respect to the common source line 45. Thus, the common source line 45 is disposed between two ground select gate lines 33g. Contacts 51, which are connected with bit lines 55, are formed in upper and lower end portions of the cell area forming drain regions of the string select gate lines 33s. 
FIG. 2 shows a cross-section taken along line I—I in FIG. 1 and FIG. 3 shows a cross-section taken along line II—II in FIG. 1.
Referring to FIG. 2, the common source line 45 is formed on the substrate in contact a portion of the active region forming common source regions 35s′ of the ground select gate lines 33g (shown in FIG. 1) and a portion of the isolation layer 23 therebetween. The bit lines 55 are disposed above the common source line 45 on an interlayer insulating layer 49.
Referring to FIG. 3, the active region 22 is not shown upward and downward as in FIG. 1, but leftward and rightward. On the active region 22, the gate lines 33g, WP, 33s are formed to cross the active region 22. The common source line 45 is in contact with the common source regions 35s′ between two ground select gate lines 33g. 
In a process of forming a cell area of a flash memory device shown in FIG. 1 to FIG. 3, first an isolation insulating layer 23 is formed on a substrate 20 to define an active region by means of a general shallow trench isolation (STI) process. The active region comprises a plurality of line shaped sub-regions. Thereafter, a gate insulating layer 24 is formed in the active region. Then, a plurality of gate lines comprising string select gate lines 33s. a plurality of word lines WP, arid ground select gate lines 33g are formed to cross the active region. Also, source/drain regions 35′, are formed to be overlapped with a plurality of line shaped sub-regions of the active region by doping an impurity on the exposed surface of the substrate between the gate lines. The source/drain regions 35′, formed by general ion implantation processes of using the gate lines and spacer 37 on both side walls of the gate lines as a mask, form a dual doped structure. Namely, highly doped portions are formed in the active region of the substrate between the adjacent spacers 37, and lightly doped portions in the active region of the substrate between the gate lines and the highly doped portions, i.e., in the active region of the substrate under the spacers 37. Then, an interlayer insulating layer 41 is deposited and planarized. Thereafter, a groove is formed to expose the common source regions 35s′ between the ground select gate lines 33g and filled with a conductor such as a polysilicon layer to form a common source line 45. Then, after an interlayer insulating layer 49 is formed over the resultant substrate, contact holes are formed to expose drain regions 35d′ of the string select gate lines 33s, and are then filled with a conductive layer to form bit line contacts. And then, bit lines are formed.
In order to decrease line resistance of the gate lines, a metal silicide layer containing metal such as cobalt (Co) or titanium (Ti) can be formed on upper portions of the gate lines as shown in black in FIG. 3. At this time, the metal silicide layer is also formed on the substrate in the source/drain regions 35′. Therefore, break down is possible due to voltage in transistor channels between the source/drain regions under the gate lines, since in a high integrated NAND type flash memory device, width of the gate lines and distance between the gate lines are very minute, for example below 0.15 μm. Particularly, in case the source/drain regions are highly doped, the transistor channels are more apt to break down since in a subsequent annealing process, the doped area is more diffused, so that the length of the transistor channels is not maintained at a proper level. In this case, a leakage of current into the substrate may also occur. Therefore, the higher the integrated degree of the elements in the memory device is, the lighter the source/drain regions have to be doped. Also, in case the silicide layer is formed in the source/drain regions, conductivity of the source/drain regions is increased, so that problems such as the break down and the current leakage become more intensified.